Selective signalling of later reserve location memory fault in compound compare and swap

ABSTRACT

A sequentially performed implementation of a compound compare-and-swap (nCAS) operation has been developed. In one implementation, a double compare-and-swap (DCAS) operation does not result in a fault, interrupt, or trap in the situation where memory address A 2  is invalid and the contents of memory address A 1  are unequal to C 1 . In some realizations, memory locations addressed by a sequentially performed nCAS or DCAS instruction are reserved (e.g., locked) in a predefined order in accordance with a fixed total order of memory locations. In this way, deadlock between concurrently executed instances of sequentially performed nCAS instructions can be avoided. Other realizations defer responsibility for deadlock avoidance to the programmer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to coordination amongst distributed computational systems and, in particular, to techniques for performing compound operations on memory in linearizable form.

2. Description of the Related Art

Computer systems often provide primitive instructions or operations that perform compound operations on memory in a linearizable form (i.e., as if atomically). For example, the VAX computer architecture provided instructions to directly support insert and delete operations on double-ended queues. Other processor architectures have provided simpler operations, such as a “test-and-set” operation (e.g., as provided by the IBM 360), a “fetch-and-add” (e.g., as provided by the NYU Ultracomputer), a “load locked” and “store conditional” operation pair (e.g., as provided by the DEC, now Compaq, Alpha), or “compare-and-swap” (e.g., as provided by the Motorola 68000 and processors conforming to the SPARC™ architecture).

SPARC architecture based processors are available from Sun Microsystems, Inc, Mountain View, Calif. SPARC trademarks are used under license and are trademarks or registered trademarks of SPARC International, Inc. in the United States and other countries. Products bearing SPARC trademarks are based upon an architecture developed by Sun Microsystems.

A “compare-and-swap” operation (CAS) typically accepts three values or quantities: a memory address A, a comparison value C, and a new value N. The operation fetches and examines the contents V of memory at address A. If those contents V are equal to C, then N is stored into the memory location at address A, replacing V. Whether or not V matches C, V is returned or saved in a register for later inspection (possibly replacing either C or N, depending on the implementation). All this is implemented in a linearizable, if not atomic, fashion. Such an operation may be notated as “CAS(A, C, N)”.

A more powerful and convenient operation is “double compare-and-swap” (DCAS), which accepts six values: memory addresses A₁ and A₂, comparison values C₁ and C₂, and new values N₁ and N₂. The operation fetches and examines the contents V₁ of memory at address A₁ and the contents V₂ of memory at address A₂. If V₁ equals C₁ and V₂ equals C₂, then N₁ is stored into the memory location at address A₁, replacing V₁, and N₂ is stored into the memory location at address A₂, replacing V₂. Whether or not V₁ matches C₁ and whether or not V₂ matches C₂, V₁ and V₂ are returned or saved in registers for later inspection. All this is implemented in a linearizable, if not atomic, fashion. Such an operation may be notated as “DCAS(A₁, A₂, C₁, C₂, N₁, N₂)”.

The SPARC version 9 architecture supports an implementation of CAS instruction on both 32-bit-wide and 64-bit-wide operands, but does not provide a DCAS instruction. On the other hand, the Motorola 68040 processor supports a double compare-and-swap instruction (on the 68040 it is called “CAS2”). Unfortunately, the CAS2 instruction effectively locks the entire memory system rather than locking first one location and then another. The net result is that deadlock is not possible but CAS2 instructions executed by separate processors are always serialized in time even if they access distinct memory locations.

There is substantial theoretical literature on the use of CAS operations and ordinary DCAS operations in the design of non-blocking algorithms. For example, Massalin and Pu proposed a collection of DCAS-based concurrent algorithms. They built a lock-free operating system kernel based on the DCAS operation offered by the Motorola 68040 processor, implementing structures such as stacks, FIFO-queues, and linked lists. See e.g., H. Massalin and C. Pu, A Lock-Free Multiprocessor OS Kernel, Technical Report TR CUCS-005-9, Columbia University, New York, N.Y. (1991).

Greenwald, a strong advocate of DCAS, built a collection of DCAS-based concurrent data structures, some of which improve on those of Massalin and Pu in terms of their properties and performance. In addition, he proposed various implementations of DCAS in software and hardware. See e.g., M. Greenwald, Non-Blocking Synchronization and System Design, Ph.D. thesis, Stanford University Technical Report STAN-CS-TR-99-1624, Palo Alto, Calif. (1999).

A drawback of the DCAS operation (as implemented by the Motorola 68040 processor and as described in the literature) is that it requires both of memory addresses, A₁ and A₂, to be valid. If either of the memory addresses A₁ or A₂ is invalid, then a memory fault occurs, typically resulting in an interrupt or trap. For certain kinds of algorithms that could otherwise profitably make use of the DCAS operation, such behavior is undesirable. For example, under certain conditions, the memory address A₂ might be invalid or unusable, but only when the contents of memory address A₁ are unequal to C_(1.)

SUMMARY OF THE INVENTION

Accordingly, it has been discovered that a sequentially performed implementation of a double compare-and-swap (DCAS) operation can be defined, which does not result in a fault, interrupt, or trap in the situation where memory address A₂ is invalid and the contents of memory address A₁ are unequal to C₁. More generally, a sequentially performed compound compare-and-swap (nCAS) operation or other compound synchronization operation can be defined. In some realizations, memory locations addressed by a sequentially performed nCAS instruction are reserved (e.g., locked) in a predefined order in accordance with a fixed total order of memory locations. In this way, deadlock between concurrently executed instances of sequentially performed nCAS instructions can be avoided. Other realizations defer responsibility for deadlock avoidance (e.g., to the programmer).

In general, two or more storage locations are accessed by compound synchronization operations in accordance with the present invention. Ordered locking of storage locations, if provided, is in accordance with any fixed total order of storage location. For example, ascending memory address order or descending memory address order are both suitable fixed total orders. While realizations in accordance with the present invention typically provide coordination between concurrently executed instances of a particular compound synchronization instruction (e.g., a DCAS), coordination between differing instructions (e.g., between 2-way and 3-way sequentially performed nCAS instructions) or between differing instruction types (e.g., between an nCAS instruction and a corresponding sequentially performed compound synchronization operation of some other type) may be desirable in some implementations. Indeed, the use of a compare-and-swap synchronization primitive is merely illustrative. Other realizations in accordance with the present invention may build on other synchronization or access primitives, e.g., read-modify-write, read-compute-conditionally write, test-and-set, etc. Similarly, while the exemplary sequentially performed DCAS and nCAS operations described herein employ the same type of access primitive in each leg thereof (i.e., a compare-and-swap), other realizations need not. For example, sequentially performed compound synchronization operations are envisioned in which earlier- and later-reserved storage locations may be accessed using different access primitives.

In some implementations, locking of memory locations is provided on a cache line basis. In others, other reservation schemes (e.g., per location locking) may be provided. More generally, reservation may include any implementation-appropriate mechanism for obtaining some form of exclusive access or rights with respect to the reserved object. In some realizations of a compound synchronization operation in accordance with the present invention, the signaling of a fault corresponding to a later reserved memory location depends on a value read from an earlier reserved memory location. For example, in some realizations of an nCAS operation, signaling of a fault corresponding to a later reserved memory location depends on successful comparison of a value read from an earlier reserved memory location with a corresponding test value.

Processors, instruction sets, computer systems and computer program products implementing and/or exploiting such sequentially performed compound compare-and-swap (nCAS) operations are all envisioned. These and other realizations will be better understood based on the description and claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings.

FIG. 1 depicts a cache-coherent shared memory multiprocessor suitable for some realizations of the present invention.

FIG. 2 depicts operation of a processor executing a sequentially performed double compare-and-swap instruction in accordance with some realizations of the present invention.

FIG. 3 depicts a flow chart of operations in accordance with an exemplary implementation of a sequentially performed double compare-and-swap instruction in accordance with some realizations of the present invention.

The use of the same reference symbols in different drawings indicates similar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 depicts an exemplary cache-coherent shared memory multiprocessor configuration. Plural processors 101 share a memory space 102. Each processor 101 implements a sequentially performed compound synchronization operation, such as a sequentially performed double compare-and-swap (DCAS) or compound compare-and-swap operation (nCAS), in accordance with the present invention. While a variety of sequentially performed compound synchronization operation implementations are envisioned, the description that follows focuses on an exemplary sequentially performed double compare-and-swap instruction (DCAS). The description of particular sequentially performed DCAS and nCAS implementations is illustrative and should not be taken as limiting. Indeed, based on the description thereof, persons of ordinary skill in the art will appreciate a wide variety of suitable implementations and exploitations, including processor, instruction set, computer system and computer program product implementations and exploitations, of this and other sequentially performed compound synchronization operations.

In general, a coherent presentation of memory space 102 and its component storage locations is maintained using any suitable techniques and/or coherence protocols. In the illustrated configuration, interactions between processors 101 and memory space 102 are mediated by respective caches 103. Accordingly, in an exemplary realization in accordance with FIG. 1, “cache-coherent” means that, from the perspective of each processor and its associated memory cache, the shared memory is logically divided into portions of, for example, 128 bytes (called “cache lines”) and that the system includes a memory controller protocol that allows each processor's cache either to “own” a cache line or to “share” a given cache line. Only one processor cache at a time may own a cache line, but any number of processors may share a cache line that is not owned. When a processor cache acquires ownership of a cache line, a copy of the data in that cache line is delivered to the cache of that processor. A processor whose cache owns a cache line is permitted to read and write the data for that cache line using the copy in its cache. If a processor whose cache owns a cache line changes the copy of the data in its cache, then the updated data must be transferred back to the shared memory at some point before ownership is released. A processor whose cache shares a cache line is permitted to read its copy of the cache line data but not to update it.

During execution of most instructions, the caches of respective processors acquire and release sharing or ownership of cache lines automatically as necessary to accomplish the reading and writing of data that may be requested by the processors. However, it is also possible for a processor to request its cache to acquire ownership of a cache line and to refuse to yield ownership until its processor directs otherwise. Such a request from a processor to its cache is called “locking” a cache line, and permission from the processor to its cache to yield ownership of a locked cache line is called “unlocking” the cache line.

FIG. 2 illustrates operation of a processor 101 in response to a sequence or set of instructions 210 that includes a sequential DCAS instruction 211. Operation 221 of processor 101 includes interactions with two locations 231, 232 of memory space 230. In general, locations 231 and 232 may be associated with the same or differing cache lines. As typically represented, the sequential DCAS instruction accepts six operands: two memory addresses A₁ and A₂ corresponding to locations 231 and 232, respectively, two comparison values C₁ and C₂, and two new values N₁ and N₂. Although a variety of definitions are possible, the values of operands A₁, C₁, N₁, A₂, C₂ and N₂ are typically stored in registers (not shown) of processor 101 and identified by corresponding register specifiers. As will be described in greater detail below, interactions with locations 231 and 232 include reservation (281, 282) of the locations (or cache lines associated therewith), read access (283, 284) to contents of the locations, and write access (285, 286) to contents of the locations. For purposes of illustration, the description that follows assumes that memory is byte-addressed, and that CL is an integer such that 2 to the power CL is the number of bytes in a cache line.

Sequentially Performed DCAS

Operation of processor 101 in response to sequential DCAS instruction 211 is now described with reference to FIG. 3. For simplicity of illustration, one major branch of the implementation (namely that corresponding to the situation where A₁ and A₂ correspond to distinct cache lines and where A₁<A₂ in some fixed total order of memory locations) is detailed in FIG. 3. Other branches (e.g., where A₂<A₁ and where A₁ and A₂ correspond to the same cache line) are summarized or omitted and will be understood in the context of the textual description.

In response to a sequential performed DCAS instruction, DCAS(A₁, A₂, C₁, C₂, N₁, N₂), processor 101 behaves as follows:

-   1. First, branch (301) based on the relationship between addresses     A₁ and A₂. If the addresses A₁ and A₂ are identical except possibly     for the low CL bits (such as if the identified memory locations     belong to the same cache line), set SHARED to true and go to step 2.     Otherwise, if A₁ is less than A₂, set SHARED to false and go to     step 3. Otherwise, set SHARED to false and go to step 5. Note that,     for simplicity, the branch to step 2 is not illustrated in FIG. 3.     The branch to step 3 and related flows are detailed in FIG. 3, while     the branch to step 5 and related flows are summarized (302). -   2. Attempt to lock the cache line that includes the memory words     identified by addresses A₁ and A₂. If the cache line is being shared     or is owned by another processor, the request will stall until the     cache line becomes available. If it is necessary to interrupt this     processor during a stall, then the program counter is left pointing     to the sequential DCAS instruction, as if execution of this     instruction had not commenced. If the memory address A₁ is invalid,     then no cache line ownership is obtained and a memory fault is     signaled in the customary manner for address A₁, resulting in a trap     or interrupt that terminates execution of the instruction. If the     memory address A₂ is invalid, then set A2OKAY to false; otherwise     set A2OKAY to true. Go to step 7. -   3. Attempt to lock (303) the cache line that includes the memory     word identified by address A₁. If the cache line is being shared or     is owned by another processor, the request will stall (304) until     the cache line becomes available. If it is necessary to interrupt     (305) processor 101 during a stall, then the program counter is left     pointing to the sequential DCAS instruction, as if execution of the     instruction had not commenced. If the memory address A₁ is invalid,     then no cache line ownership is obtained and a memory fault is     signaled (307) in the customary manner for address A₁, resulting in     a trap or interrupt that terminates execution of the instruction.     Continue to step 4. -   4. Attempt to lock (308) the cache line that includes the memory     word identified by address A₂. As before, if the cache line is being     shared or is owned by another processor, the request will stall     (309) until the cache line becomes available. If it is necessary to     interrupt (310) this processor during a stall, then the cache line     that was locked at 303 is unlocked (311) and the program counter is     left pointing to the sequential DCAS instruction, as if execution of     this instruction had not commenced. If the memory address A₂ is     invalid, no reservation is performed and no memory fault is     signaled, but A2OKAY is set (312) to false. Otherwise A2OKAY is set     to true (313). Go to step 7. -   5. Attempt to lock the cache line that includes the memory word     identified by address A₂. If the cache line is being shared or is     owned by another processor, the request will stall until the cache     line becomes available. If it is necessary to interrupt this     processor during a stall, then the program counter is left pointing     to the sequential DCAS instruction, as if execution of this     instruction had not commenced. If the memory address A₂ is invalid,     no reservation is performed and no memory fault is signaled, but     A2OKAY is set (314) to false. Otherwise A2OKAY is set to true.     Continue with step 6. -   6. Attempt to lock the cache line that includes the memory word     identified by address A₁. If the cache line is being shared or is     owned by another processor, the request will stall until the cache     line becomes available. If it is necessary to interrupt this     processor during a stall, then the cache line that was locked in     step 5 is unlocked and the program counter is left pointing to the     sequential DCAS instruction, as if execution of the instruction had     not commenced. If memory address A₁ is invalid, then no cache line     ownership is obtained and a memory fault is signaled (315) in the     customary manner for address A₁, resulting in a trap or interrupt     that terminates execution of the instruction. Continue with step 7. -   7. Fetch (316) from the cache of processor 101 the contents V₁ of     the word at memory address A₁. Continue with step 8. -   8. Compare (317) V₁ to C₁. If they differ, then replace N₁ with V₁     and replace N₂ with zero (318), and go to step 13. Otherwise     continue with step 9. -   9. If A2OKAY is false, signal (319) a memory fault for address A₂ in     the customary manner, resulting in a trap or interrupt that     terminates execution of the instruction. Otherwise, continue with     step 10. -   10. Fetch (320) from the cache of processor 101 the contents V₂ of     the word at memory address A₂. Continue with step 11. -   11. Compare (321) V₂ to C₂. If they differ, then replace N₁ with V₁     and replace N₂ with V₂ (322), and go to step 13. Otherwise continue     with step 12. -   12. Store (323) to the cache of processor 101 the value N₁ into the     memory word at address A₁ and likewise store (323) N₂ into the     memory word at address A₂. Replace N₁ with V₁ and replace N₂ with     V₂. Continue with step 13. -   13. Unlock (325) the cache line that includes the memory word     identified by address A₁. If SHARED is false and A2OKAY is true,     unlock (326) the cache line that includes the memory word identified     by address A₂. Terminate execution of the instruction.

Based on the description herein, persons of ordinary skill in the art will appreciate that steps 1 through 6, above, reserve locations associated with addresses A₁ and A₂ in an order such that if the memory locations identified by these addresses belong to two different cache lines then the lower address is reserved first. This technique avoids a deadlock situation in which two separate processors each attempt to reserve two memory locations P and Q, but one processor reserves P first and then Q while the other reserves Q first and then P, allowing the possibility of each processor succeeding at its first reservation request and then stalling, waiting for the other processor to release its reservation.

While the above description has presumed use of cache line locking, other reservation mechanisms may also be employed. For example, locking may be performed on an individual storage location basis. Similarly, although memory address order (e.g., ascending or descending) is simple and multiple memory locations can be efficiently ordered in accordance therewith, other ordering techniques are also possible. In general, any fixed total order of storage locations employed by respective instances of sequentially performed compound synchronization operations is suitable.

In some realizations, deadlock avoidance need not be provided by the implementation of the sequentially performed DCAS instruction. For example, in one such realization, steps 5 and 6 (above) may be omitted and step 1 is updated to change the words “step 5” to “step 3.” Accordingly, the DCAS implementation itself no longer discriminates based on the ordering of A₁ and A₂ with respect to some fixed total order of memory locations. Deadlock is a possibility and it is, instead, up to the programmer to use the DCAS instruction carefully to avoid deadlock. For example, code employing such a modified DCAS instruction may employ preceding instructions to ensure that operands are supplied to the DCAS instruction in accordance with a fixed total order of memory locations. In any case, other advantages of the DCAS implementation remain. For example, a memory error is not signaled on account of address A₂ if V₁ does not match C₁.

In some realizations, a sequentially performed DCAS instruction may also deliver a Boolean (single-bit) result to indicate succinctly whether or not data was successfully stored to memory. For example, steps 8 and 11 (above) may be suitably augmented by inserting “store FALSE into the Boolean result register” before the words “go to step 13.” Similarly, step 12 (above) may be suitably augmented by inserting “Store TRUE into the Boolean result register” before the words “Continue with step 13.”

Sequentially Performed nCAS

While a double compare-and-swap instruction serves as a useful example, persons of ordinary skill in the art will appreciate that the techniques described herein may be extended to other instructions including instructions that act upon a larger number of memory locations. For example, a sequential compound compare-and-swap (nCAS) accepts n memory addresses A[j] (1<=j<=n), n comparison values C[j] (1<=j<=n), and n new values N[j] (1<=j<=n).

Although other implementations are possible, it is simplest if a memory reservation on an invalid address never causes a memory fault. The operation of an nCAS instruction (n>=2) is as follows:

-   1. Grade the memory addresses in numerical order (as if by the     “grade up” operation of Iverson's APL programming language) so as to     produce a grade vector G that is a permutation of the integers from     1 to n such that, for all 1<=j<n, A[G[j]]<=A[G[j+1]]. -   2. Repeat the following for all values of j in order from 1 to n:     -   If j=1, set SHARED[j] to false. If j>1 and memory addresses         A[G[j−1]] and A[G[j]] both identify memory locations that belong         to the same cache line, then set SHARED[j] to true and copy         AOKAY[G[j−1] into AOKAY[G[j]]. Otherwise, set SHARED[j] to false         and ask this processor's cache to lock the cache line that         includes the memory word identified by address A[G[j]]. If the         cache line is being shared or is owned by another processor, the         request will stall until the cache line becomes available. If it         is necessary to interrupt this processor during a stall, then         (first) for all values of k in order from 1 to j−1, if SHARED is         FALSE and AOKAY[G[j]] is TRUE, the cache line that includes the         memory word identified by address A[G[k]] is unlocked; and         (second) the program counter is left pointing to the sequential         nCAS instruction, as if execution of this instruction had not         commenced. If the memory address A[G[j]] is invalid, no         reservation is performed and no memory fault is signaled, but         AOKAY[G[j]] is set to false. Otherwise AOKAY[G[j]] is set to         true. -   3. Repeat the following for all values of j in order from 1 to n:     -   If AOKAY[j] is false, signal a memory fault for address A[j] in         the customary manner, resulting in a trap or interrupt that         terminates execution of the instruction. Otherwise, fetch (from         this processor's cache) the contents V[j] of the word at memory         address A[j]. Then, compare V[j] to C[j]. If they differ, then         set L to j−1, terminate the repetition of this step, and go to         step 6. -   4. Repeat the following for all values of j in order from 1 to n:     -   Store (to this processor's cache) the value N[j] into the memory         word at address A[j]. -   5. Set L to n. -   6. Repeat the following for all values of j in order from 1 to L (if     L equals zero, do not perform this step at all):     -   Replace N[j] with V[j]. -   7. Repeat the following for all values of j in order from L+1 to n     (if L equals n, do not perform this step at all):     -   Replace N[j] with zero. -   8. Repeat the following for all values of j in order from 1 to n:     -   If SHARED[j] is false and AOKAY[G[j]] is true, unlock the cache         line that includes the memory word identified by address         A[G[j]].

Note that nCAS never locks any given cache line more than once and that it locks them in ascending address order. In one variation, each occurrence of “SHARED[j]” (in steps 2 and 8, above) may be replaced by “SHARED[G[j]]”.

As before, the sequential nCAS instruction may be augmented to deliver a Boolean (single-bit) result to indicate succinctly whether or not data was successfully stored to memory. For example, step 3 (above) may be augmented by inserting “store FALSE into the Boolean result register” before the words “go to step 6”, and step 5 may be augmented by inserting “Store TRUE into the Boolean result register.” before the words “Set L to n.”

Also as before, operation of a modified sequential nCAS instruction may omit the ordering of memory addresses before locking the cache lines and instead lock in the order in which the addresses are presented. For such a modified instruction, it is as if the vector G were always treated as consisting of the integers from 1 through n in order, rather than as a vector that indicates how to sort the vector A of addresses. As before, the burden of deadlock avoidance may be shifted to the programmer.

While the above description of sequentially performed DCAS and nCAS instructions has presumed that signaling of faults corresponding to a later reserved memory location is entirely dependent on success of an access to an earlier reserved memory location, variants of the sequentially performed DCAS or nCAS instruction may distinguish more than one category of invalid or unusable memory address and signal (or not) accordingly. For example, a memory fault of one category may be selectively signaled in accordance with the outcome of comparison operations as outlined above, whereas a memory fault of another category is always signaled regardless of the outcome of the comparison operations. In one exploitation, presentation of a memory address that is a multiple of 4 (i.e., properly aligned), but for which there is no entry in a virtual page table might trigger a memory fault only if the memory contents of all preceding addresses equal their corresponding memory values, whereas presentation of a memory address that is not a multiple of 4 could always signal a memory fault.

While the invention has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them. Many variations, modifications, additions, and improvements are possible. For example, compare-and-swap synchronization is only one suitable primitive form. Other compound synchronization operations may employ (i) fault signaling for an access to a later reserved storage location, which depends on an access to an earlier reserved storage location, and/or (ii) reservation of such storage locations in accordance with a fixed total order thereof. Such compound synchronization operations may employ other access primitives such as test-and-set, read-modify-write, read-compute-conditionally write, etc. In addition, reservation of storage locations may be performed by locking associated cache lines or otherwise, such as by locking other groupings or by locking storage locations individually. Some realizations need not employ cache memory as an intermediary or locking facility. In general, any fixed total order of storage locations, including ascending and descending memory address orders, may be employed. Although certain exemplary compound synchronization instructions have been described that operate on shared byte-addressable memory, variations that employ other shared storage locations such as shared registers are also envisioned.

More generally, plural instances may be provided for components, operations or structures described herein as a single instance. Finally, boundaries between various components, operations and data stores are somewhat arbitrary, and particular operations are illustrated in the context of specific illustrative configurations. Other allocations of functionality are envisioned and may fall within the scope of claims that follow. Structures and functionality presented as discrete components in the exemplary configurations may be implemented as a combined structure or component in other configurations. These and other variations, modifications, additions, and improvements may fall within the scope of the invention as defined in the claims that follow. 

1. A method of operating a computer system that includes first and second processors and memory shared thereby, the method comprising: concurrently executing first and second instructions on respective ones of the first and second processors, the first and second instructions each attempting to reserve in a same predefined order plural respective locations of the memory, wherein, for at least the first instruction, signaling of a fault corresponding to a later one of the respective locations depends on a value read from an earlier reserved one of the respective locations.
 2. The method of claim 1, wherein the predefined order is in accordance with a fixed total order of locations within the memory.
 3. The method of claim 1, wherein the predefined order is one of ascending and descending memory address order.
 4. The method of claim 1, wherein the reserving includes locking an associated cache-line.
 5. The method of claim 1, wherein the reserving locks at least the respective location, but substantially less than all the memory.
 6. The method of claim 1, wherein the first and second instructions are linearizable synchronization operations.
 7. The method of claim 1, wherein the first instruction is a double compare-and-swap instruction.
 8. The method of claim 1, wherein the first instruction is a compound compare-and-swap instruction; wherein the respective locations reserved by the compound compare-and-swap instruction number N; and wherein signaling of a fault corresponding to a later reserved one of N locations depends on at least one value read from an earlier reserved one the N locations.
 9. The method of claim 1, wherein the first instruction is a double compare-and-swap instruction; and wherein, unless a value read from the earlier reserved one of the respective locations compares to a corresponding test value, no fault corresponding to the later reserved one of the respective locations is signaled.
 10. The method of claim 1, wherein the execution of the first instruction includes access to the earlier reserved one of the respective locations; and wherein, unless the access succeeds, no fault corresponding to the later reserved one of the respective locations is signaled.
 11. The method of claim 1, wherein the execution of the first instruction includes a first access corresponding to an earlier reserved one of the respective locations; and an optional second access that signals a fault only if the first access succeeds.
 12. The method of claim 1, wherein the respective locations reserved by the first and second instructions are disjoint; and wherein the concurrent execution is non-blocking.
 13. A method of operating a computer system that includes a memory shared by plural processors thereof, the method comprising: in response to execution of a single instruction by one of the processors, separately attempting to reserve plural locations of the memory prior to accessing contents of any one of the successfully reserved memory locations; and signaling a fault corresponding to a later one of the locations based on a value read from an earlier reserved one of the locations.
 14. The method of claim 13, wherein, unless the value read from the earlier reserved location compares to a corresponding test value, no fault corresponding to the later reserved location is signaled.
 15. The method of claim 13, wherein the separately reserving includes separately locking at least the plural locations, but substantially less than all the memory.
 16. The method of claim 13, wherein the computer system further includes cache storage; and wherein the separately reserving includes separately locking respective cache lines associated with the plural locations.
 17. The method of claim 16, wherein the cache storage includes a coherently maintained set of caches including ones respectively associated with each of the plural processors.
 18. The method of claim 13, wherein the instruction implements a compound Compare-and-swap (nCAS) operation.
 19. The method of claim 13, wherein the instruction implements a Double Compare-and-swap (DCAS) operation.
 20. The method of claim 13, wherein at least one of the plural locations is identified by an operand of the instruction.
 21. A method of operating a processor, the method comprising: in response to execution by the processor of a single instruction that separately accesses plural memory locations, separately attempting to reserve a first and second one of the plural memory locations prior to accessing contents of the successfully reserved memory locations; accessing the first one of the memory locations; and selectively signaling a fault corresponding to the second one of the memory locations depending on a result of the first memory location access.
 22. The method of claim 21, further comprising: in response to the execution of the single instruction, detecting the fault as part of the second memory location reservation; and signaling the fault, if at all, only upon success of the first memory location access.
 23. The method of claim 21, wherein the first memory location access includes comparing a value read from the first memory location to a corresponding test value; and wherein the signaling is performed, if at all, only upon success of the comparing.
 24. The method of claim 21, further comprising: signaling the fault corresponding to the second memory location, if at all, based on a value read from the reserved first memory location.
 25. A processor that implements an instruction that separately attempts to reserve a first and second memory locations prior to accessing contents of either of th first and second memory locations, and that addresses first and second memory locations but for which signaling of a fault corresponding to the second memory location depends on a value read from the first reserved memory location.
 26. The processor of claim 25, wherein, unless the value read compares to a test value, no fault corresponding to the second memory location is signaled.
 27. The processor of claim 25, wherein the instruction is a compound compare-and-swap instruction.
 28. The processor of claim 25, wherein the fault, if any, corresponding to the second memory location is detected upon reservation thereof, but signaled, if at all, based on the value read from the reserved first memory location.
 29. A computer program product encoded in at least one computer readable medium, the computer program product comprising: a set of instructions executable on a processor, the set of instructions including at least one instance of an instruction directing the processor to separately reserve first and second memory locations prior to accessing contents of either of the first and second memory locations, and to access first and second memory locations but for which signaling of a fault corresponding to the second memory location depends on a value read from the first memory location.
 30. The computer program product of claim 29, wherein the fault, if any, corresponding to the second memory location is detected upon reservation thereof, but signaled, if at all, based on the value read from the reserved first memory location.
 31. The computer program product of claim 29, wherein the instruction further directs the processor to reserve the first and second memory locations in a predefined order in accordance with a fixed total order of memory locations.
 32. The computer program product of claim 29, wherein, unless the value read compares to a test value, no fault corresponding to the second memory location is signaled.
 33. The computer program product of claim 29, wherein the instruction is a compound compare-and-swap instruction.
 34. The computer program product of claim 29, wherein the at least one computer readable medium is selected from the set of a disk, tape or other magnetic, optical, or electronic storage medium and a network, wireline, wireless or other communications medium.
 35. An apparatus comprising: a memory store; means for separately attempting to reserve, in response to a single instruction, first and second locations of the memory store prior to accessing, in response to a single instruction, contents of the first and second locations of the memory store if successfully reserved; and means for signaling, if at all, a fault corresponding to the second location based on a value read from the reserved first location. 